In a conventional through-hole mounting structure, terminals of an electronic component are inserted in holes drilled in a printed circuit board and soldered to both walls of the holes and lands formed around openings of the holes by solder. Thus, the electronic component is electrically connected to the printed circuit board. Since the terminals of the electronic component pass through the through holes and are exposed to the back surface of the printed circuit board, other electronic components cannot be mounted to the back surface area having the though holes. Therefore, the through-hole mounting structure limits circuit density of the printed circuit board.
In a blind-hole mounting structure disclosed in JP-H10-41605A, terminals of an electrical component are inserted in blind holes drilled in a printed circuit board. Since the terminals of the electronic component are not exposed to the back surface of the printed circuit board, other electronic components can be mounted to the back surface area having the blind holes. Therefore, the blind-hole mounting structure allows high circuit density of the printed circuit board compared to the through-hole mounting structure.
However, the blind-hole mounting structure reduces contact area between the terminals and solder compared to the through-hole mounting structure. Therefore, temperature change (i.e., difference in the linear coefficient of expansion) or application of external force causes a large stress per unit area on junctions between the terminals of the electronic component and the lands of the printed circuit board. The large stress damages the junctions so that connection reliability between the terminals and the lands may be reduced.
In a conventional surface mounting structure, terminals of an electrical component are bent along a front surface of a printed circuit board and soldered to lands formed on the front surface of the printed circuit board. Thus, the electrical component is electrically connected to the printed circuit board. Since electronic components can be mounted to each side of the printed circuit board, the surface mounting structure allows high circuit density of the printed circuit board.
However, in the surface mounting structure, the terminals of the electronic component are placed on the lands of the printed circuit board and then soldered to the terminals in a reflow process. Therefore, the terminals may be displaced from the lands before the solder hardens. For example, the displacement may occur when the circuit board or the electronic component is deformed due to the difference in the linear coefficient of expansion between the circuit board and the electronic component. Further, it is difficult to ensure coplanarity of the lands because the number of the terminals is large. Therefore, the connection reliability between the terminals and the lands may be reduced.